A common problem in captive breeding groups of rhesus macaques is high rates of contact aggression and wounding. Even though rhesus monkeys in the wild also show frequent aggression as a means of reinforcing dominance relations between group members, best practices in managing captive rhesus dictate that we attempt to understand and reduce aggression. Past research has shown that increasing the proportion of non-natal males in breeding groups is a promising method for significantly improving the group stability, welfare, and production in captive rhesus colonies. These non-natal adult males, particularly high ranking ones, intervene most frequently and most successfully in conflict among their group members, leading to lower rates of aggression and trauma and improved social stability. The primary goal of the proposed project is to increase the ratio of non-natal adult males in breeding groups, and to determine which factors are the most critical in achieving that goal. By collecting extensive behavioral data before, during, and after the introduction of new male cohorts to large breeding groups, the long-term effects of the resulting sex ratios on group stability, chronic stress, and reproduction can be assessed. The specific aims are: (1) to identify factors that predict a male's ability to successfully integrate nto a new breeding group, including comparing different techniques to introduce new males to a group, characteristics of the males (e.g., personality, social experience, success in conflict intervention), and breeding group characteristics (e.g., degree of matriline fragmentation, behavioral patterns, group size); (2) to determine the effects of different introduction techniques and male conflict intervention success on social network measures of group stability by creating aggression and dominance rank networks, and by measuring cortisol values in hair samples as a representation of perceived stress among group members across study phases; and (3) to evaluate interrelationships among sex ratio, group stability, wounding rates, stress, and reproductive success. We predict that as the proportion of non-natal adult males in groups increases, group stability measures will improve. This research will help to improve the welfare and breeding success of rhesus macaques held as an animal resource to support biomedical and behavioral research by providing healthy animal models for this work. This research is also responsive to the societal expectation for promoting and protecting animal welfare.